The general objective of this research is to determine the structure and function of the mammalian nucleoside diphosphate hexose pyrophosphorylases, or more specifically: 1. Physical and chemical characterization of the crystalline calf liver enzyme. 2. Some of the unique structural features of the calf liver enzyme will be further explored and compared with the same enzyme from other sources. 3. The catalytic properties of the pyrophosphorylase from human tissue will be examined in detail because of the possible importance of this enzyme in galactosemia.